Hemeproteins have been studied for decades using synthetic models and by studying hemeproteins directly. Much of the effort using model systems has been in studying various distal effects; proximal effect have largely not been systematically studied beyond the T-state models 2- methylimidazone and 1,2-dimethyl imidazole. We proposed to investigate the kinetic and thermodynamic consequences of proximal effects on ligand binding to iron porphyrin systems. We propose to characterize the binding of a series of a substituted imidazole bases to ferrous heme model systems, and to determine the magnitude of the effect on the binding of the imidazoles and on the trans effects these substituted imidazoles may have on the binding of CO, O2 and NO ligands. Wee propose a series of equilibrium constant determinations and millisecond-to-microsecond kinetic analyses of these hemes using a series of 2-alkylimidazoles as proximal bases. Our specific aims are 1) Test proxima-base interaction on ligand-heme-based complex stability, including estimating the electronic contribution of imidazole base binding by measuring imidazole acidities; 2) Test proximal-base effects on ligand-heme-base kinetics, including the development of a YAG-based kinetics apparatus; 3) Test of nitric oxide preference to 5-coordinate or 4- coordinate heme, including stopped-flow and slower kinetics. Through characterizing the influence of proximal strain on these processes, we hope to provide a more complete picture as to proximal effects on ligand binding to hemes and hemoproteins.